The invention relates to a suspension system for a bicycle and is intended for incorporation into the frame of the bicycle to improve comfort, safety and performance. As will be apparent from the description which follows, the suspension system will find application in other wheeled structures, such as motorized cycles, wheelchairs, and wherever suspension is needed.
In order to incorporate a suspension system into a bicycle, the bicycle frame must have two relatively movable frame components. Normally, a first component is associated with a front wheel or forward portion of the cycle and a second component is associated with a rear wheel or rearward portion of the cycle. The suspension system is installed between the frame components and includes a shock absorber, whereby impacts to the frame are absorbed or directed in some way to minimize impacts to the rider.
A common form of shock absorption in a bicycle comprises a coiled compression spring mounted to a shaft coupling the components of the frame and bearing against a piston. The nature of the spring can be selected to provide lesser or greater resistance to impacts and the piston may optionally be associated with a hydraulic or pneumatic device, or an actuator to prestress the spring or modify the spring rate of travel. There is however a very limited range of travel for the spring and little flexibility for adjusting the sensitivity of the assembly in accordance with the weight of the rider and the nature of the terrain over which the rider anticipates riding the bicycle.
The disadvantages of a shock absorber incorporating a coiled compression spring with limited axial displacement as described above have been recognized in the motorcycle industry. U.S. Pat. No. 2,596,411 to Jordan provides a suspension system in which a torsion bar couples the front steering forks of a motorcycle and includes a shaft mounted to arms extending from the front wheel axis. A rubber-like member is secured to the shaft and is adapted to isolate the steering forks from the front wheel. A hydraulic, adjustable link couples one end of the arms remote from the torsion bar to the steering fork to supplement the resiliency in the torsion bar.
Similarly, in U.S. Pat. No. 5,487,443 to Therm, a torsion bar extends between frame struts on the mainframe. The torsion bar is fixed to the main frame at one end and is rotatably connected to the main frame at the other end on a xe2x80x9cfloating nutxe2x80x9d or bushing. A torsion bar arm and turnbuckle link couple the torsion bar to a rear frame or swing arm for mounting a rear wheel. Preferably, an auxiliary damper to absorb linear shock is disposed between the mainframe and the rear frame.
Such torsion bars cannot be readily accommodated into a bicycle frame without substantial modification to the frame. Further, they exhibit limited sensitivity and cannot be adjusted to suit a rider""s requirements and individual preferences for a firm suspension or a softer suspension.
U.S. Pat. No. 4,913,255 to Takayanagi et al provides a shock absorber for a motorcycle incorporating a rotary-type hydraulic damper and a turnbuckle linking an arm coupled to the shaft of the hydraulic damper and to a swing arm or rear frame for supporting a rear wheel.
The patent provides for varying the speed of rotation of the rotary-type oil bath hydraulic damper by changing the length of the turnbuckle link and thereby adjusting the damping power characteristic of the damper. However, the rotary oil bath hydraulic damper is costly and complex to produce. The suspension system is also undesirably cumbersome and adds significant weight to the cycle.
Applicant""s invention is intended to overcome the aforementioned disadvantages in a simple and effective manner.
In its simplest embodiment, the invention provides a hydraulic damper comprising a cylinder of polyurethane material mounted to a toothed shaft which is adapted to be fixed to a first portion of a cycle frame. The polyurethane damper is confined by a casing bearing on an outer cylindrical surface of the damper, and by transverse bearings bearing on opposite ends of the damper and adapted to prevent axial displacement and flow of the damper. Advantage is being taken of the unique properties of polyurethane when placed under hydrostatic pressure.
Preferably, the casing has an arm extending in a radial direction to the axis of the shaft and the arm may be segmented into two portions, whereby the effective length of the arm can be adjusted. Advantageously, the resulting torque applied to the damper for a predetermined force can be varied to change the damping characteristics of the damper.
A link which couples the arm to a second portion of the cycle frame may be adjustable in length so as to maintain the cycle geometry, if desired, in any selected configuration of the arm portions. Alternatively, a bracket for supporting the shaft and slidable on the cycle frame is provided to compensate for changes in cycle geometry.